The present invention relates to an automatic draft control plate which is capable of maintaining efficient draft conditions in the operation of a furnace. The plate is provided with a horizontal counterbalance, which is used to initially set the plate in a partially open position, and a vertical adjustment to vary the sensitivity of its movements in accordance with changes in ambient conditions.
Gas, coal and oil burning furnaces are very prevalent in our society and have many industrial, commercial, and residential applications. For example, it is common in industrial plants for large furnaces to be utilized in conjunction with a heat exchanging device to heat a particular material as an essential step in the processing of that material. Thus, in the refining of oil to produce gasoline and other petroleum products, crude oil must be heated in large outdoor furnaces as a part of the refining or "cracking" process. Thereafter, hydrogen or other byproducts of the cracking process must be again heated in similarly large furnaces. Thus, it is common in oil refineries for as many as 25 to 50 oil or gas-fired furnaces to be in constant 24-hour operation.
As in a typical furnace, an oil refinery furnace consists of a firebox, a heat exchange section, and a tall stack or flue to permit the escape of the gaseous byproducts of the furnace's operation. Combustion occurs in the firebox where fuel, fed into the burners at a predetermined rate, is mixed with air drawn in at the base of the furnace. The burners typically have draft regulators which, to a certain extent, can be used to control the amount of air or "draft" being mixed with the fuel. Furthermore, these large refinery furnaces sometimes have a firebox and heat exchange section which is as large as 10 to 20 feet in diameter and a stack of up to 200 feet in height. The temperature in the heat exchange section often reaches 1500 degrees Fahrenheit. These furnaces consume massive amounts of oil and natural gas, fuels which have become extremely expensive in the past few years. In fact, with around-the-clock operation of these furnaces, the annual fuel costs incurred in operating each one of these furnaces can exceed $200,000. Therefore, small improvements in the efficiency of the operation of a furnace can yield large savings in fuel costs.
The efficiency of a refinery furnace is determined by the ability of the furnace to heat the crude oil flowing through it to the desired temperature while using a minimum amount of fuel. As is well known in the art, the maximum amount of heat which can be derived from the consumption of a specific amount of fuel occurs when there has been "perfect combustion". Perfect combustion occurs when the ingredients of that combustion, fuel and air, are completely consumed and only the gaseous byproducts of combustion, such as carbon dioxide and water vapor, remain. Non-perfect combustion is inefficient because the existence of excess combustion ingredients deprives the flame of heat and requires the consumption of additional fuel in order to heat the crude oil to the desired temperature.
Thus, the efficiency of the furnace's operation can be determined by measuring the amount of fuel or air which is not consumed in the combustion process.
In order to avoid inefficient excess oxygen conditions in the byproducts of the furnace's combustion, it is essential that the amount of air entering the burners of the furnace not be excessive. The entrance of this air, commonly termed the "draft" of the furnace, is caused by the rapid ascent of the hot gaseous byproducts of combustion. These gaseous byproducts are much less dense and have a much higher temperature than the ambient air surrounding the furnace. They therefore rise rapidly through the flue of the furnace where they are mixed with the cooler outside air. The rising flue gases create a partial vacuum or a low pressure area in the firebox of the furnace, causing convection currents of outside air, which is at ambient atmospheric pressure, to be drawn into the furnace. These convection currents constitute the "draft" of the furnace, and if they exceed the amount needed for perfect combustion, the furnace will be operating inefficiently.
As is well known in the art, the amount of draft, i.e., the volume of air entering the burners, depends upon pressure differential between ambient pressure and the low pressure in the firebox caused by the rising flue gases. The higher the velocity of these gases, which is a function of their temperature, the lower the pressure in the firebox and in turn the greater pressure differential. Thus, the velocity of the draft, and in turn the volume of draft air entering the furnace, increases or decreases, respectively, with an increase or decrease in the velocity of the flue gases.
In contrast to the problem of excess oxygen caused by high draft velocity, perfect combustion can also be prevented by the presence of excess fuel, or in other words insufficient draft velocity. When this condition occurs, the fuel fed into the burners is not completely consumed in the combustion, as evidenced by the presence of dark or black smoke coming from the stack. The smoke is darkened because it consists not only of the gaseous byproducts of the combustion but also of minute carbon particles and other particulates. The presence of this heavier particulate matter slows the velocity of the byproducts of the combustion as they travel up the stack. Furthermore, as with excess combustion oxygen, these particulates deprive the combustion process of heat, cooling the surrounding combustion gases, and further decreasing their velocity. Therefore, an insufficient amount of draft air enters the furnace and the problem of fuel-rich combustion is compounded. As a result, the furnace produces less than the maximum amount of heat and fuel consumption must be increased.
Thus, draft conditions in a furnace are critical for maintaining its efficient operation and the conservation of fuel.
Although perfect combustion, or zero percent excess or deficient oxygen, is an ideal goal for furnace combustion, this condition is not practical to attain in practice. Air leaks through holes and other openings in the furnace, faulty burner and air regulator operation, and other contributory problems have made 3-5 percent excess oxygen a practical goal for efficient furnace operation. Therefore, the efficiency of the furnace can be conveniently monitored by measuring the oxygen content in the gaseious byproducts of combustion and maintaining it in this range.
Heretofore, improper draft control has been the major contributing factor to inefficient furnace operation, making even this goal of 3-5 percent of excess oxygen difficult to achieve. As discussed above, changes in the draft velocity may be caused by controllable factors, such as the amount of fuel fed into the burners. For example, an increase in the furnace's fuel consumption produces oxygen deficient combustion, resulting in dark smoke, less heat generation and lower flue gas velocity. Consequently, draft velocity also decreases and the oxygen deficiency is compounded.
Inefficient draft conditions can also be caused by several uncontrollable factors, consisting primarily of changes in ambient conditions. For example, a wind blowing across the top of the stack will create a low-pressure area at that point, increasing the velocity of flue gases and that of the draft as well. It should also be pointed out that winds at the top of tall refinery furnace stacks are usually much stronger than at the base of the furnace. Thus, it is common for such winds to create excess oxygen conditions in the furnace which reach 10 to 12 percent.
Also, a decrease in temperature of the surrounding air will increase its density. Therefore, if the velocity of the draft remains unchanged, a condition of excess oxygen will result since the volume of air entering the furnace contains more oxygen molecules. Furthermore, if atmospheric pressure increases, the pressure differential between ambient and the firebox is greater, increasing the velocity of the draft. Again, the amount of oxygen being drawn into the furnace becomes excessive.
Each of these changes in fuel supply or ambient conditions affect the velocity and/or amount of draft, thereby changing the combustion characteristics of the furnace and decreasing the efficiency of its operation. That is, each change results in either excessive or insufficient oxygen conditions in the furnace. However, as explained above, these changes also affect the velocity of the flue gases. For example, a large increase in the fuel supply to the furnace, as discussed above, can result in smoke, cooler combustion, and a lower flue gas velocity. As just mentioned, a wind blowing over the stack of the furnace also increases the velocity of the gases rising in the flue.
Therefore, it is common to compensate for these changes in draft intensity by providing the furnace with a damper plate in the flue to control the velocity of the flue gases. That is, if the draft velocity should become excessively high, increasing the desirable amount of excess oxygen, the damper can be adjusted to partially obstruct the flue. This obstruction decreases the flue gas velocity and, in turn, decreases the velocity and amount of draft air drawn into the furnace. Thus, the amount of excess oxygen is reduced to a permissible range. On the other hand, if an increase in draft conditions is desirable, the damper can be adjusted to permit the unobstructed passage of flue gases, increasing the draft velocity.
Typically, these flue dampers are manually operated and therefore must be adjusted with each change in operating conditions. Thus, this method of draft control is very disadvantageous since it increases the labor costs associated with operating the furnace or if sufficient manpower is unavailable, the furnace is permitted to operate inefficiently. Furthermore, a very common problem associated with these flue dampers is that, with time, they become inoperative because their bearings deteriorate and stick as a result of their exposure to the extremely hot gases flowing in the flue. Thus, even if manpower is available to properly adjust these dampers, they cannot be used for efficient draft control.
It is also well known to provide the furnace with a channel, in communication with the main flue, in which an auxiliary damper is mounted. This type of auxiliary damper installation has the advantage of removing the damper from the direct path of the hot gases in the flue. Although these auxiliary dampers are also used to regulate the velocity of the flue gases, their operation is different from that of dampers mounted in the main flue of the furnace. That is, if the draft velocity increases undesirably, rather than obstructing the flow of gases in the main flue in order to inhibit the entrance of air into the burners of the furnace, outside air is drawn into the flue through the auxiliary damper and channel. This cooler outside air mixes with the hot flue gases, reduces their temperature and velocity, and, in turn, reduces the velocity of the draft.
Outside air is drawn in through these auxiliary dampers much like draft air is drawn into the burners. The velocity of flue gases creates a low static pressure inside the stack which is less than ambient atmospheric pressure. Thus, outside air is drawn into the flue in response to this pressure differential. An auxiliary "draft" is established into the flue which can be used, by adjusting the position of the auxiliary damper plate in the channel, to control the main draft through the furnace burners. If, for efficiency reasons, this main draft should be decreased, the damper plate is open to permit the flow of outside air; if the main draft should be increased, the plate is closed so that the flue gas velocity as well as the draft velocity will be increased.
It has also been found that these auxiliary dampers, to a limited extent, can be mounted to provide automatic draft control under certain conditions. For example, an undesirable increase in the draft velocity will result from an increase in the velocity of the flue gases, thereby lowering the static pressure in the stack. Prior art dampers are constructed and mounted so that they will automatically open, permitting outside air to flow into the flue, in response to the increased pressure differential between ambient air and the flue gases. This cooler outside air reduces the temperature and velocity of the flue gases and, in turn, the velocity of the draft air, restoring efficient operating conditions.
However, auxiliary damper plates of the prior art, although somewhat automatic in controlling draft intensity, are not sufficiently sensitive to changes in draft conditions. For example, prior art plates will not open to permit the flow of outside air until the pressure differential across the plate is sufficient to overcome its own weight. Therefore, until the velocity of the flue gases is sufficient to establish such a pressure differential, inefficient draft conditions are permitted to persist. On the other hand, a plate whose weight is only barely sufficient to maintain it in a closed position flaps wildly in response to large changes in pressure differential. As a result, draft conditions may be lower or higher than desirable for efficient operating conditions. Therefore, auxiliary dampers of the prior art are not sensitive to the changes in draft conditions which often occur.
A further substantial disadvantage associated with auxiliary dampers of the prior art is that they normally operate in a closed position, totally preventing outside air from entering the flue. Under these conditions, if the velocity of the draft should be increased, such as for example when more fuel is burned, the auxiliary damper is unable to permit more air to be drawn in at the base of the furnace. Thus, insufficient oxygen conditions persist, resulting in inefficient furnace operation.
Thus, the prior art has not met the need for an automatic draft controller which is capable of increasing as well as decreasing the velocity and amount of the draft according to changes in operating conditions of the furnace. Furthermore, there is a need for an automatic draft controller which can be adjusted to provide appropriately sensitive movements in response to changes in the draft of the furnace.